is therefore recommended that the maximum trip current should be significantly (lower than Isc stc MAX). But the overcurren recommended (1.4Isc) How ￼it can
Talk:PV System: how to ensure safety during normal operation
Incorence or clarification needed - paragraph "Protection against overcurrent"
Some modifications have been made on the content of the page PV System: how to ensure safety during normal operation after the comment received. More detailed explanations are included in the discussion below, for those who would be interested.
- we modified the sentence saying "...significantly (lower than Isc stc MAX)" : the point that we want to highlight is that, because short-circuit currents on the PV installation may be lower than the (closest upstream) overcurrent protection, it is recommended to take specific protection measures to disconnect the circuit in case of short-circuit. Which might be uneasy as these are very low short-circuit levels.
- we removed the sentence saying "... protection rating higher than 1.4 times the protected string ...", because it was not clear. The idea we wanted to convey is that it might be better to choose an overcurrent protection rating a little bit higher than the minimum value requested by the standard (>1.25 or >1.5 times ISC) to avoid the risk of nuisance tripping
Thanks for your valuable comments!
nota: I also just modified the title of this title, to make it more clear for other users. I hope you will be ok with the title I chose.
Good afternoon, as the previous commenter pointed out, both points were contradictory, and personally I think, they continue to be so right now. How can a device offer protection against overcurrent> 1.25 /> 1.5 times Isc and at the same time be able to cut a current lower than those same values?
What specific protection measures can be taken to disconnect a short-circuited string or sub-matrix, which are complementary to overcurrent protection in case of reversed currents?
Thank you for your attention, receive a cordial greeting from Spain.
Hi @Manuel, here is the answer from the expert:
The formulae for string and sub-array overcurrent protection (1.5 and 1.25 times I<sub>SC</sub> are given by IEC 60364-7-712 edition 2017.
Indeed in case of low irradiance the overcurrent protection may not detect an issue in a faulty string or sub-array (for example, in a faulty string, the reverse current corresponds to the total current produced by all other strings, which may still be lower than the overcurrent protection of the faulty string in case of low irradiance. And in that case the overcurrent protection will not trip until the irradiance and thus the reverse current are high enough).
This situation is acceptable because the sizing of conductors and the withstand value of PV modules should be selected accordingly.
So I believe it's the statement "It is therefore recommended to disconnect faulty circuits in case of short-circuit" that you see as contradictory?
We should probably remove this recommendation that is not in IEC standards and also because we don't provide any solution to achieve that.
Would that clarify things for you?
Hi @LMischler Thank you for your answer, I am only a learner in this matter and I have many doubts about it. I appreciate that you have taken the time and trouble to answer my questions. I interpret from your answer that it will not be possible to disconnect a chain or sub-arrangement with a short-circuit current (current in its direction), but only those inverted chains, since their inverse current exceeds the current of the protective device associated with that chain in inverted state. It makes sense because the current at the maximum power point is only a little less than the short-circuit current, so the unexpected trips of the supposed "doubtful" device would be recurrent, this makes it impossible to protect the chains themselves from their short-circuit currents. Thanks again.